Ion implantation is a technique for introducing conductivity-altering impurities into a substrate. A desired impurity material is ionized in an ion source and extracted. The extracted ions are then manipulated into an ion beam and accelerated to a prescribed energy, and directed toward the substrate and implanted. After annealing the substrate the ions are disposed in the substrate's lattice to form a region having a desired conductivity.
Solar cells are only one example of a device that uses silicon substrates. They provide pollution-free, equal-access energy using a free natural resource, and they are becoming more important globally. Any reduced cost to the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
Solar cells may require doping to improve efficiency. This doping may either be blanket doping of the entire solar cell surface of selective doping where only specific areas of the solar cell surface are doped. In the past, solar cells have been doped using a dopant-containing glass or a paste that is heated to diffuse dopants into the solar cell. The process, however, does not provide sufficient precision to allow selective doping of the various regions of the cell. Moreover, if voids, air bubbles, or contaminants are present, non-uniform doping may occur during blanket doping.
Doping may improve cells where a surface of the solar cells includes a grid of conductors that collect photocurrent. Increasing the dopant dose under these grid lines will reduce the series resistance and increase the solar cell efficiency. Increasing the dose globally or across the entire surface through blanket doping, however, will instead increases surface recombination and lower the efficiency of the solar cell. Therefore, the ability to dope a series of narrow stripes would be beneficial to solar cell production.
Solar cells could benefit from ion implantation because ion implantation allows precise doping of the solar cell and enable selective doping. Selective doping of solar cells, however, may require a certain pattern of dopants of that only certain regions of the solar cell substrate are implanted with ions. Previously, implantation of only certain regions of a substrate has been accomplished using photoresist. Use of photoresist, however, would add an extra cost to solar cell production because extra process steps are involved. This also poses a difficulty if the regions to be implanted are extremely small. Accordingly, there is a need in the art for improved implanting of solar cell substrates and, more particularly, improved implanting of solar cell substrates using a mask.